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should be in the ordered conformation to promote interaction. This
was then questioned, indicating that disorder was required. The current
understanding is that heterotypic (xanthan-galactomannan) as well as
homotypic (xanthan-xanthan) interactions are required for gelation with
the heterotypic junction zones being formed with xanthan in the disor-
dered conformation. This model is based on viscosity evidence from
low concentration solutions prepared from two stock solutions mixed
in different ratios. The technique was developed from the early work
of Cuvelier and Launay (1988), but instead of selecting the two stock
solutions based on polymer concentration, the technique used was to
adjust their zero shear specific viscosity to 1 (twice the viscosity of
water). When mixed in different ratios, departure from the zero shear
specific viscosities of the two stock solutions provided direct indication
of interaction. Polymer-exclusion and phase-separation phenomena are
negligible at the low polymer concentrations involved, and therefore,
such events are not the origin of the observation, as they would be for
systems of higher polymer concentrations. Examples of polymer con-
centrations of xanthan and LBG required to give such starting stock
solutions are 0.007 and 0.06%, respectively, with a 60:40 blend pro-
ducing a sevenfold increase in the zero shear specific viscosity (Foster,
1992). In the latter study, it was also shown that deacetylation of xan-
than (DX) promoted the synergistic interaction with LBG, due to the
destabilisation of the ordered helix, which promoted the formation of
heterotypic junction zones and resulted in a 100-fold increase in vis-
cosity. On the other hand, for xanthan lacking the terminal mannose
unit in its side chain, and therefore the destabilising pyruvate group, the
helix was more stable and resisted heterotypic junction zone formation;
as a result no interaction was observed across the blending ratios used
(Foster and Morris, 1994; Morris and Foster, 1994). A change of the
co-synergist to konjac glucomannan (KM) showed a 30-fold and 500-
fold increase in the viscosity of xanthan and DX, respectively (Foster,
1992). Thus, comparison of the efficiency of the interaction, relating to
the conformation or type of co-synergist, provides not only the funda-
mental understanding of the nature of the interaction but also the tools
for optimising it.
The method was developed further to screen for molecular interac-
tions in mixed hydrocolloid systems, and in addition, it has been applied
to measure stoichiometry of interaction (Goycoolea
et al.
, 1995a). Fur-
ther measurements of stoichiometry of interaction were based on rheo-
logical measurements to provide evidence of heterotypic binding. The
gel's storage modulus (
G
) and the enthalpy of transition showed a lin-
ear increase with increasing KM:DX to 1:1, with little further change at
higher ratios. It was also found that tan
goes through a sharp minimum
at similar compositional ratios, indicating efficiency in the usage of the
δ
